Update "Tails based on Debian testing" plan

wiki/src/blueprint/Debian_testing.mdwn

@@ -137,6 +137,9 @@ part of a broader community.
 
 # The plan
 
+**Update**: this was deferred. We'll come back to this in 2018-04 or
+2018-05 and either come up with a new plan, or postpone this entirely.
+
 * From now to the end of 2017-11: the Foundations Team tries to port
   the code & test suite during sprints. If the work doesn't fit into
   these sprints then we'll need to draw conclusions.